Separation of the elements of air



May 2, 1961 D. L. SMITH 2,982,107

SEPARATION OF THE ELEMENTS OF AIR Filed Dec. 16, 1957 INVENTOR.

Donald Leslie Smirh ATTORNEY 7 224; fig; ,a nf

Unitizd Sims tent 2,982,107 SEPARATION OF THE ELEMENTS OF AIR Donald Leslie Smith, Berkeley Heights, NJ., assignor to All Reduction Company, Incorporated, New York, N.Y., a corporation of New York Filed Dec. 16, 1957, Ser. No. 703,196

7 Claims. (Cl. 62-22) This invention relates to the separation of the elements of air by liquefaction into high purity liquid prodnote, and more particularly relates to an improved procgen and recycle nitrogen and so impurities, such as carbon dioxide, are deposited and removed from the air. Upon reversal of the flow paths of the waste nitrogen and the incoming air; the waste nitrogen effectively removes the deposited substances because proper temperature levels are provided in the reversing exchanger by means of a suitable bleed-01f and return of some of the refrigerating recycle nitrogen from the nonreversing passage of the exchanger to the nitrogen recycle exchanger.

The refrigerated incoming air is next scrubbed by rich liquid in the lower portion of the high pressure column Where the liquid collecting at the base air is boiled by high pressure recycle nitrogen and a purge fraction containing krypton and xenon is removed from the bottom of the scrubber along with impurities.

Thereafter the scrubbed air is rectified into high purity nitrogen eflluent, and oxygen-enriched liquid. air (rich liquid) in the upper portion of the high pressure column. The nitrogen efliuent is partially 'condensed by heat exchange with high purity liquid oxygen in a separate re-.

boiler which is connected to the bottom of the oxygen column; Condensation of the nitrogen effluent from the nitrogen column is completed in a separate nitrogen condenser which is refrigerated by expanded recycle nitrogen from the boiler of the scrubber. This condensation step results in the separation of a gaseous off-take which contains constituents of air which are more volatile than nitrogen, principally neon. Part of the condensed cfiluent is returned to the top of the nitrogen column as reflux in order to produce high purity nitrogenwithout theme of a large number of vapor-liquid contact plates.

After completing the condensation of the nitrogen efiluent, the expanded recycle nitrogen is divided and utilized for refrigerating the incoming air (as mentioned).

and for refrigerating the compressed recycle nitrogen in the recycle nitrogen exchanger. Part of the compressed nitrogen is removed from the warmpassage of the re-. cycle exchanger and is cooled by passing through an ex-. pansion engine. This expanded recycle nitrogen is then passed through the cold passage of the recycle exchanger ice - 2 from the reversing exchanger in order that proper heat transfer conditions can be maintained.

The product from the high pressure column which is processed further is the rich liquid which collects in the bottom of the upper portion of the high pressure column.

The rich liquid is expanded and then rectified in the oxygen column and the oxygen reboiler connected thereto into waste nitrogen and high purity liquid oxygen. This rectification is effected by means of the reflux liquid nitrogen and the high pressure, high purity nitrogen effiuent which boils the high purity liquid oxygen in the reboiler.

At the proper point in the oxygen column, an argon concentrate containing principally argon and oxygen is passed to the argon column or attachment for separation into crude argon and residual liquid oxygen. This separation is effected in part by passing the expanded rich liquid from the high pressure column prior to its admission to the oxygen column through the argon condenser in the top of the argon attachment where the'rich liquid condenses the reflux for the argon column. Before en-' tering the condenser of the argon column, the rich liquid is utilized to liquefy the gaseous crude argon from the argon column so that the argonproduct is in a form which is more suitable for small volume storage.

The process for obtaining high argon recovery according to this invention broadly involves the introduction of a high purity'nitrogen reflux liquid to the top of the low pressure oxygen column, maintaining the oxygen content of the waste nitrogen efliuent from the low pressure oxygen column below about 0.015 mol percent and the argon content of the m'trog'en efliuent below about 0.15 molpercent, and adjusting the amount of the high purity nitrogen reflux liquid delivered to the low pressure oxygen column to at least 38 mol percent of the total incoming air to produce a strata at an intermediate level in the oxygen column in which the argon concentration in the vapor phase is in excess of about 10 mol'percent.

The requirements of this invention with respect to the purity of the nitrogen reflux liquid, the amount of nitrogen reflux liquid, the oxygen and argon content of the waste nitrogen eflluent,.and the maintenance of a strata at an intermediate level in the oxygen column in which the argon concentration in the vapor phase is inexcess of about 10 mol percent are not independent but are interrelated and cooperative.

The nitrogen reflux liquid introduced into the low pressure oxygen column must be a high purity nitrogen liquid and must contain less oxygen and argon than would be in equilibrium with the waste nitrogen eflluent. This can be accomplished by maintaining the oxygen content of the nitrogen reflux liquid at about 0.05 mol percent this invention. An amountof nitrogen reflux liquid in in order to refrigerate the remainder of the compressed left the warm passage, by the recycle nitrogen bleed-oil excess of 38 'mol percent can be used; however, nitrogen reflux liquidof about 50 mol percent of the total air ,feed is the most that can be used or obtained by utilizationpf the liquefaction'process herein described. Increasing the amount of nitrogen reflux liquid results in a corresponding increase in the percentage of argon recovery andalso a corresponding increase in the concentration of; the argon at an intermediate level in the oxygen column, The concentration of the argon at an intermediate level; in the oxygen column can exceed; and preferably does mol percent in the vapor phase. The maximum concentration of argon at an intermediate level in the oxygen column which can be obtained by utilizing the liquefaction process disclosed herein is approximately 20 mol percent. It has been foundhowever .that it is advantageous-to drawofi the argonin the vapor form fromthe oxygen column for further rectification having a concentration of between about 12 to 15 mol percent. This minimum concentration of the argon at an intermediate level in the oxygen column between the pool of liquid oxygen in the bottom and the waste nitrogen effluent is obtained by controlling the amount and purity of the nitrogen liquid reflux and the oxygenand argon content in the waste nitrogen efiluent and'also by, as will be readily appreciated by those skilled in the art, control of the number of plates usedin the column together with the rate and amount of liquid or gaseous oxygen withdrawal at the bottom.

In addition to the cooperation and relationship be tween the steps of the process for obtaining a high argon recovery involving the purity and amount of liquid nitrogen reflux in the low pressure column, the oxygen and argon concentration in the waste nitrogen effluent, and the argon concentration in the low pressure column, there is a definite and advantageous cooperation between this method of operation of the low pressure column as above described and the particularmethod of liquefaction ofthe, incoming air as-described herein involving the refrigeration of the incoming air by a separate nitrogen cycle and the delivering of the air feed to the high pressure column at saturation. This cooperation resides in the inherent ability of such a liquefaction system to make available under normal operating conditions the required amount of high purity nitrogen liquid reflux of at least 38 mol percent of the incoming air at the low pressure column.

This invention thus also includes a novel and. advantageous overall process for the separation of the elements of air which broadly comprises refrigerating the incoming air-by means of a separate nitrogen cycle, delivering the air to the high pressure nitrogen column at saturation, separating the air into a high purity nitrogen efiluent and an oxygen enriched liquid, condensing the high purity nitrogen eflluent into a high purity liquid nitrogen, delivering the oxygen enriched liquid to the low pressure oxygen column for rectification, delivering the liquid nitrogen-to the top of the low pressure oxygen'column as nitrogen reflux in an amount equivalent to'at least 38 mol percent of the incoming air, separating the oxygen enriched liquid into waste nitrogen effluent at'the top of the column, a liquid oxygen product at the bottom, and an argon rich strata intermediate the waste nitrogen and oxygen containing at least about 10 mol percent argon in the vapor phase, and maintaining the oxygen content of the waste nitrogen eflluent below about 0.015 mol percentand the argon content below about 0.15 mol percent and withdrawing the argon from the low pressure column at the point where its concentration is at least about 10 molpercent in the vapor phase for delivery to an argon column to recover the argon.

The instant invention and the process and apparatus for practicing the same will be described in more detail and will be more readily apparent from the 'following description and the appended drawing.

' Referring to the schematic drawing, it can be seen that reference numeral '11 is appliedtothe air inlet conduit. Conduit 11 passes air to compressor 13 where the air is compressed to about 100 p.s.i.a. Thereafter the compressed air is cooled to about 80 F. by conventional coolers '(not shown). The compressed air after flowing from compressor 13 through conduit 15,reversing valve 17, andconduit 19 enters passage 21 ofreversing exchanger 23. In flowing through passage 21'the air is cooled to approximately its liquefaction temperature (about -280 F.) by heat exchange with counter-current flowing streams of waste nitrogen in passage 25 and of recycle nitrogen in passage 27 of reversing heat exchanger 23. Exchanger 23 is comprised of two reversing passages 21 aud 25 and one nonreversing passage 27 and is suitably designed to eflect eflicient indirect heat, exchange between the respective streams of air, waste nitrogen, and recycle nitrogen. In accordance with the illustrated setting of valve '17, the waste nitrogen is shown as leaving passage 25 of exchanger 23 by means of conduit 28, passing through valve 17, and being discharged to the atmosphere. As is conventional in the art, means are provided to reverse or alternate the flow of air and waste nitrogen in the passages 21 and 25 in order that removal of carbon dioxide and other impurities which are deposited from the incoming air can be accomplished by the waste nitrogen in addition to the indirect cooling of the air by waste nitrogen. This waste nitrogen is derived in the separation process in a manner to be explained hereinafter.

The means for reversing the flows of waste nitrogen and air in passages 25 and 21 comprises the reversing valve 17, a timing system (not shown), and suitable check valves 29 in the various conduits leading from and .to the cold end of the reversing exchanger 23. The timing system can be any conventional means suitable for properly operating the reversing valve 17 on a predetermined time cycle and, in the interest of clarity, has not been shown.

Two of the previously-mentioned check valves 29 are located in the two conduits 31 and 33 which are shown respectively as passing air from exchanger 23 towards the separation process and as bringing waste nitrogen from the separation process to the exchanger 23. A branch conduit 35 having a check valve 29 joins waste nitrogen conduit 33upstreamlfrom its check valve and extends to and joins the air conduit 31 downstream from its check valve. Another branch conduit 36 provides part of the alternate flow path for air from a point downstream of the check valve in conduit 33 to a point upstream of the check valve in conduit 31. With this arrangement it is apparent that air can alternately flow through passages 21 and 25 of the exchanger upon reversal of valve 17 and that waste nitrogen flows in the passage not being used by air.

After being cooled in exchanger 23, the air vapor flows to the high pressure column 37, enters the lower scrubber section 39 thereof where the air is at about 94 p.s.i.a. and passes up through conventional contact trays countercurrent to oxygen-enriched liquid air. This scrubbing operation removes any traces of high boiling constituents orimpurities from the air, which traces may have passed through exchanger 23. The scrubber liquid in the bottom of the scrubber is boiled or revaporized by the fluid inboiler 41 and a small portion of the'scrubber liquid having the impurities concentrated therein is purged from the bottom of the scrubber 39 by means of valved conduit 43. It is to be noted that by the action which occurs in scrubber 39, krypton and xenon in the air are removed therefrom and pass out of scrubber 39 in the purge liquid passing through conduit 43, rather than flowing on through the process. This purge can be suitably treated to obtain the krypton and xenon, if desired, as a separated rare gas product.

Air vapor from the scrubber 39 passes up into the nitrogen rectifier section 45 of the high pressure column 37 where it is rectified into high purity nitrogen effluent by means of conventional contact trays and the liquid nitrogen reflux which enters the top of the nitrogen rectifier 45 through pipe 47. Due to this rectification, liquid air enriched in oxygen (rich liquid) collects in annular trough '49 in the bottom of rectifier section 45. Part of this liquid is used for the above-mentioned scrubbing operation and enters the scrubber 39 by means of valved pipe 51 extending from a conduit connected tothe top level of the trough 49 to the scrubber v39.

passes..:through conduit 51 and'is used in scrubber 39,

this rich fluid is passed through conduit 55, its expansion valve 57, the argon apparatus (including the reflux condenserof argon attachment 1'40) and conduit 58 tothe, upper portion of the oxygen column 59. This oxygen enriched air. is'fed at about 18 p.s.i.a. and is rectified in oxygen column 59, having the conventional tray-contact construction, into waste nitrogen vapor in the top and a pool of liquid oxygen in the bottom. This liquid oxygen at about -291.F. is withdrawn with the aid of gravity from the bottom of rectifier 59 by means of conduit 61 and ispassed to the oxygen reboiler 63 where it is partially reboiled to give high purity liquid oxygen. The vapor which is thus formed is returned to oxygen column 59 by means of conduit 65 which empties into the oxygen column adjacent the bottom but above the pool of liquid oxygen therein. The net liquid oxygen produced in re.- boiler 53 is removed therefrom by conduit 67 and a suitable liquid oxygen pump 69 as the high purity liquid oxygen product of the process.

' The rectification which occurs in oxygen column 59-is- 5 refluxed by the above-mentioned liquid nitrogen; This liquid nitrogen is drawn from the line 47 and flows through line 53 to subcooler 75 where it is subcooled by waste nitrogen from the oxygen column prior to its refluxing. By conduit 77 this subcooled liquid nitrogen isypassedto expansion valve .79 where this liquid is expanded to about 18p.s.i.a: and then it is introduced into thetop' of column 59 as reflux.

1 The waste nitrogen vapor at about --317 F. from the top of oxygen column 59 passes through conduit 81 to subcooler 75 where it effects the above-mentioned subcooling of the liquid nitrogen which refluxes the oxygen column. Thereafter this waste nitrogen at about -290 F. flows by pipe 33 to the reversing exchanger 23 to effect the previously described refrigeration of incoming air. v

"Considering now the high purity nitrogen at about 2 85 which is produced in the top of the nitrogen column 37, it can be seen on the drawing that a conduit 83'1extends from'the top of high pressure column 37 to nitrogen is transferred by pipe 87 to the nitrogen con- 50.

denser 89; Condensation ofthis nitrogen is completed in'condenser89 by heat being abstracted by the recyclev nitrogen at about 60 p.s.i.a. in the upper section 91 of the condenser 89. Part of this condensed liquid nitrogen is thend'elivered'for reflux to the top of high pressure portion of the recycle nitrogen flow in passage 27 is column 37 by conduit 93, liquid nitrogen pump 95, and conduit 47. Part of the liquid nitrogen at about 90 p';s.i.a. can be withdrawn as a high purity nitrogen product by m'ea ns of valved pipe 97 which connects to conduit 47 downstream of the liquid nitrogen pump 95 is this is desired. v i

, ';Pai't of this condensed liquid nitrogen is also then de- IiVered 'for reflux to the top of the low pressure oxygen column 59 through the conduit 53, the subcooler 75, and

'the conduit 77. The purity of this condensed liquid nitroen at this point is such that it contains less oxygen and argon than would be inequilibrium with the waste nitrogen eflluent containing no more than 0.015 mol percent of oxygen and 0.15 mol percent of argon. As discussed above, theamount of this condensed liquid nitrogen depressure nitrogen :tower results in the production of a high purity nitrogen at the top of the tower and an oxygen rich liquid at the bottom of the tower. The nitrogen 7 contained in the top of the column 37 is in its purest state at the very top of the column, and its purity decreases as it moves downwardly through the column. Generally, the nitrogen contained near the top of the column 37 is also of suflicient purity so that it can also be utilized as liquid nitrogen reflux for the oxygen column 59. Although the nitrogen can be withdrawn from the top of the tower 37 for use as reflux in the column 59, so long as it is Withdrawn from the column 37 at a point where it is of sufficient purity, it is preferred to withdraw the liquid nitrogen reflux for the oxygen column directly from the conduit 47 or after it has passed'through the con: denser 89.

The condenser 89 serves to separate out the neon and othersimilar gases since these gases will tend not to be condensed as is the nitrogen eflluent. Since these gases are more volatile than nitrogen, they will pass'to the top of the condenser 89 and areremoved by conduit 98. This separation of neon and other gases, besides being a worth-; while recovery, also serves to maintain a high purity nitrogen since the neon, for example, is purged from the nitrogen and the system.

As above mentioned, the incoming air is compressed to about 100 p.s.i.a. in compressor 13. At this pressure the air, Of course, does not have suflicient energy to pro vide the refrigeration required; therefore the well-known nitrogen recycle is added to the system in a particular, mannerto furnish this necessaryrefrigeration. This nitrogen recycle has been mentioned above in reference to the. scrubberboiler 41 and condenser 89. The minor flowof ,the gaseous recycle nitrogen at about 270.' F. enters this boiler'41 and, after boiling enriched liquid scrubbing air and so being liquefied passes through expansion valve 101 in conduit 102 which is connected to boiler 41 and leads to nitrogen condenser 89. After expanding through valve 101 to about 4 atmospheres gauge pressure and moving into the upper section 91.of the condenser 89, this recycle nitrogen flow is completely evaporated by condensing the nitrogen effluent entering condenser 89 by means of conduit 87. g ,After effecting the condensation oithe nitrogen efiiu ent in condenser 89, the evaporated recycle nitrogen or minor flow of nitrogen leaves condenser 89 through conduit 105 and is dividedinto' two parts by means of conduit 107 which joins conduit 105. The part of the evaporated recycle-nitrogen which'flows in conduit 107 having control valve-108 is utilized in the nitrogen recycle in a manner which will be subsequently explained. The.

residual part'of the evaporated nitrogen which flows on in conduit 105 passes to the nonreversing passage 27- of the reversing exchanger 23 to effect part of the previously mentioned cooling the incoming air, along with the waste nitrogen flowing in either passage 25 or 21. A small withdrawn through conduit 109 at a point approximately two-thirds of the Way up the exchanger from the entrance I of conduit 105. This withdrawn nitrogen passes through conduit 109 and its control valve 110 and enters passage 131 of the exchanger 123 at a point opposite where thecompressed nitrogen to be expanded leaves passage 121 for expander 127. The reason for thiswithdrawal will be explained shortly. The major or larger portion of the residual part of evaporated recycle nitrogen continues through the. passage 27 of the exchanger and leaves through conduit 111 having a control valve 113. This larger portion of recycle nitrogen next joins an augmented major flow of recycle nitrogen moving in conduit 115 which leads to the nitrogen recycle compressor 117 and thus forms a full nitrogen recycle flow. The full flow is compressed by, and discharged from compressor 117 into conduit 119 at pressure of about 2500 p.s.i.a. Conventional water-cooled heat exchangers (not shown). lower the temperature of the recycle nitrogen, to about;

80 F. Conduit 1 19 passes the nitrogen to passage 121 of the high pressure recycle exchanger 123 where the full flow is initially cooled by the augmented, expanded major flow. At a point in passage 121 where the compressed nitrogen is at a temperature of about 30 F., a major flow of the high pressure stream or full flow of nitrogen is withdrawn through conduit 125 and expanded more or less isentropically in a conventional expansion engine 127 to about 4 atmospheres gauge with the performance of external work. This cold expanded major flow is directed by conduit 129 to passage 131 of the high pressure recycle nitrogen exchanger 123 where it effects cooling of the counter-flowing compressed minor flow of nitrogen in passage 121. In this manner the minor flow of the compressed nitrogen which remains after the major flow leaves passage 121 for expansion continues through 121 and is further cooled. From passage 121, this minor flow passes through conduit 133 having valve 134 which suitably reduces the pressure of the nitrogen (preferably to about 160 p.s.i.a.), to the boiler 41 in scrubber 39 where it is further cooled as above described. As above mentioned, the throttled minor flow is divided after condenser 89 into two parts at the juncture of conduits 105 and 107. The part in conduit 107 is added to the exhaust from the expansion engine and thereby forms the augmented expanded major flow which moves through passage 131 while the residual part flows on in conduit 105 to reversing exchanger 23 and functions in the manner above explained.

The withdrawal of the small portion of the residual part of the evaporated minor flow of nitrogen from passage 27 of exchanger 23 by means of valved conduit 109 is done in order to provide, indirectly, the temperature conditions in reversing exchanger 23 which will permit proper deposition and efiective removal of high boiling impurities, such as carbon dioxide. It is to be noted that this withdrawn nitrogen passes through conduit 109 and its control valve 110 and enters the augmented expanded nitrogen passage 131 of. the high pressure nitrogen exchanger 123 at a particular point. This point of introduction corresponds to the location in the exchanger 123 at which the major flow of nitrogen is divided from the compressed full flow for delivery to the nitrogen expander'127.

Referring now to the argon attachment to the oxygen column, it can be seen that the argon attachment 140 is arranged to Withdraw an argon concentrate from the oxygen column 59 at a point about one-half of the distance up from the base of the column or at a point in the column where the argon concentration is at least about mol percent in the vapor phase. This withdrawn argon passes through passage 142 and enters the argon column 140 which also has the previously mentioned liquid vapor contact trays in the major portion thereof.

In the top of the column a condenser 144 receives expanded rich liquid produced in the high pressure column 37. By means of condenser 144 and the bubble cap tray construction, the argon concentrate is rectified into enriched-oxygen in the base of the argon column while crude or raw argon (at about -301 F.) containing a small percentage of oxygen is removed from the top of the argon column 140 by conduit 145. Conduit 145 passes the raw argon to the raw argon liquefier 147 which is also cooled by the expanded rich liquid from the high pressure column. The rich liquid for the raw argon liquefier 147 of the argon column flows from the high pressure column 37 through conduit 55 and valve 57 from annular trough 49. This rich liquid is warmed in passing through passage 149 of liquefier 147, countercurrently to the crude argon in passage 151 which is being liquefied. The liquefied argon at about -304 F. is removed by pipe 153 connected to passage 151. The rich liquid passes from liquefier 147 to the condenser 144 of the argon column by means of pipe 155. The rich fluid at about -308 F. is removed from the condenser down from the top. The oxygen which collects in the" bottom of the argon column is returned to the oxygen colunm 59 by conduit 161 which extends between the base of the argon column and a point in the oxygen column slightly below where argon concentrate is removed.

'The conduit 171 can be used to bleed off some gaseous oxygen if and when this is desired. The proper bleed-off can be controlled by automatic valve 173 in conduit 171. This valve 173 is preferably operated automatically in re sponse to adetermination of the composition of the gas at point of withdrawal. tion is made by taking a gas sample from the withdrawal location in the oxygen column 59 by means of valved conduit 175 and passing the sample to a gas analyzer 177 connected to conduit 175. The automatic valve 173 is controlled by the gas analyzer 177 by suitable means 179 shown by a dash-line on the drawing.

It is to be noted that the air separation process can be considered as being divided into a high pressure zone which includes high pressure column 37 and a low pressure zone which includes the oxygen column 59 and the oxygen reboiler 63.

The operation of the apparatus and steps of the process are believed to be apparent to one skilled in the art from the foregoing description.

The arrangement for obtaining high purity nitrogen involves passing the entire nitrogen effluent from the top of the nitrogen column 37 through pipe 83 to the nitrothen completely liquefied in nitrogen condenser 89 byheat exchange with the expanded minor flow of recycle nitrogen. Thereafter part of this large quantity of liquid nitrogen or condensed nitrogen efiluent is returned to the top of the high pressure rectifying zone or nitrogen column 37 by means of pump 95 and conduit 47 in order to reflux the rectification of air into a high purity nitrogen efiluent. A portion of the liquefied high purity nitrogen can be removed from the process after the complete condensation for the various uses which require high purity nitrogen if desired. In this manner, high purity liquid nitrogen can be produced simultaneously with high purity liquid oxygen in a plant which is basically a liquid oxygen plant and nitrogen is produced having a purity higher than 99.9 percent.

In addition to being used to complete the condensation of the high purity nitrogen effiuent and hence refrigerate to some extent the process, the nitrogen recycle is used to boil the liquid air in scrubber 39 and to refrigerate the incoming air in reversing exchanger 23 so that the advantages which are inherent in the use of a nitrogen recycle are obtained in an improved manner.

in the recycle provide a flexible and efiicient system for placing certain of the recycle flows in condition for accomplishing the foregoing boiling, condensing and refrigerating functions. It is also to be noted that by hav-' ing a closed nitrogen cycle the lubricant contaminants from the cycle compressor 117 do not get into the air flows and cause an explosion hazard and that the cycle can be operated at high' pressure. By maintaining the low side of the nitrogen cycle including the expander 127 at about 4 atmospheres, it is possible to get suitable heat transfer and yet the total power requirements of the instant process are not larger than the power required for conventional plants.

From the foregoing it is apparent that the instant invention provides an improved process for the simultaneous production of high purity liquid oxygen and a'h'igh argon recovery in an efficient and economical manner.

It is to be understood that the person skilled in the art The determination of gas composi-- It is to be noted that the various divisions and flows of the nitrogen 9 can make changes in the instant invention as herein disclosed in its preferred form without departing from the invention as defined in the following claims. I claim: 1. In an air separation process in which air is coole and liquefied in a high pressure rectifier into oxygen-rich and nitrogen-rich fractions, respectively, and wherein said oxygen-rich fraction and at least a portion of said nitrogen-rich fraction are delivered to a low pressure rectifier in which relatively high purity nitrogen and oxygen products are separated, the improvement for obtaining a high argon recovery from. the rectified air which comprises condensing the nitrogen eflluent from said high pressure rectification, separate from the reflux liquids, into a high purity liquid nitrogen containing less than equivalent to at least about 38 mol percent of the total 0.05 mol percent oxygen and less than 0.3 mol percent argon, introducing said high purity liquid nitrogen of the same purity as reflux at the top of said low pressure rectifier in an amount equal to at least about 38 mol percent of the total incoming air, removing nitrogen eflluent from the top of said low pressure rectifier and maintaining the oxygen content therein below about 0.015 mol percent and the argon content below 0.5 mol percent, maintaining an argon content in the intermediate vapors of said low pressure column of at least 10 mol percent, and withdrawing at least a portion of said intermediate vapors from said column.

2. In an air separation process which produces high purity liquid oxygen and has 'a high pressure rectifier having a set of contact trays and a low pressure rectifier having a set of contact trays, the improvement for producing high argon recovery which comprises condensing the nitrogen efliuent from said high pressure rectifier, separate from the reflux liquids, into a high purity liquid nitrogen containing less than 0.05 mol percent oxygen and less than 0.3 mol percent argon, introducing part of said condensed nitrogen efliuent of the same purity into said low pressure rectifier above the contact trays as reflux, said liquid nitrogen reflux being introduced into said low pressure rectifier in an amount equal to at least about 38 mol percent of the total incoming air, removing the waste nitrogen effluent from the top of the low pressure rectifier, and maintaining the oxygen content in said waste nitrogen effluent below about 0.015 mol percent and the argon content below about 0.15 mol percent.

3. The process of obtaining liquid oxygen and a high recovery of argon in excess of about 80 percent which comprises refrigerating air by means of a sepaarte nitrocondensed liquid nitrogen eflluent of the same purity to said low pressure column as liquid reflux in an amount incoming air and While maintaining the oxygen content in said waste nitrogen efiluent below about 0.015 mol percent and the argon content below 0.05 mol percent, and withdrawing said intermediate vapors containing at least about 10 mol percent argon from the low pressure column for further processing.

4. The process of claim 3 in which the argon concentration is maintained between about 12 to 15 mol percent. a

5. In an air separation process which produces high purity liquid oxygen and has a high pressure liquefier having a set of contact trays and a low pressure rectifier having a set of contact trays, the improvement for producing high argon recovery which comprises condensing the nitrogen eflluent from said high pressure rectifier, separate from the reflux liquids, into a high purity liquid nitrogen containing less than 0.05 mol percent oxygen and less than 0.3 mol percent argon, returning part of said condensed nitrogen effluent as reflux to said high pressure column and introducing part of said condensed nitrogen efiluent of the same purity into said low pressure rectifier above the contact trays as reflux, said liquid nitrogen reflux being introduced into said low pressure rectifier in an amount equal to at least about 38 mol percent of the total incoming air, removing the waste nitrogen efiluent from the top of the low pressure rectifier, and maintaining the oxygen content in said waste nitrogen eflluent below about 0.015 mol percent and the argon content below about 0.15 mol percent.

6. An air separation process in accordance With claim 2 wherein an argon content is maintained in the intermediate vapors of said low pressure rectifier of at least 10 mol percent in the vapor phase, and the intermediate vapor from said column is withdrawn at said intermediate point where the argon concentration is at least 10' said separate nitrogen cycle and a part of said condensed nitrogen efiluent is introduced to said low pressure column as liquid reflux and a part thereof returned to said high pressure column as liquid reflux.

References Cited in the file of this patent UNITED STATES PATENTS 1,880,981 Pollitzer et a1. Oct. 4, 1932 2,057,804 Twomey Oct. 20, 1936 2,433,508 Dennis Dec. 30, 1947 r 2,547,208 Simpson Apr. 3, 1951 2,762,208 Dennis Sept. 11, 1956 2,793,511 Bonnaud May 28, 1957 2,817,216 Etienne Dec. 24, 1957 OTHER REFERENCES Separation of Gases by Ruhemann (second edition), pages 217 to 226 relied on. 

